1. Field of the Invention
The present invention relates to a membrane electrode assembly having a seal integrated at an edge of the assembly, as well as a solid polymer electrolyte fuel cell containing such an assembly.
2. Description of the Prior Art
Fuel cell systems are currently being developed for use as power supplies in numerous applications, such as automobiles and stationary power plants. Such systems offer promise of efficiently providing power with environmental and other benefits. Fuel cells convert reactants, namely fuel and oxidant, to generate electric power and reaction products. Fuel cells generally employ an electrolyte disposed between two electrodes, namely a cathode and an anode. A catalyst typically induces the desired electrochemical reactions at the electrodes.
Preferred fuel cell types include solid polymer electrolyte (SPE) fuel cells that contain a solid polymer electrolyte and operate at relatively low temperatures. During normal operation of a SPE fuel cell, fuel is electrochemically oxidized at the anode catalyst, typically resulting in the generation of protons, electrons, and possibly other species depending on the fuel employed. The protons are conducted from the reaction sites at which they are generated, through the electrolyte, to electrochemically react with the oxidant at the cathode catalyst. The catalysts are preferably located at the interfaces between each electrode and the adjacent electrolyte.
A broad range of fluid reactants can be used in SPE fuel cells and may be supplied in either gaseous or liquid form. For example, the oxidant stream may be substantially pure oxygen gas or a dilute oxygen stream such as air. The fuel may be substantially pure hydrogen gas, a gaseous hydrogen-containing reformate stream, or an aqueous liquid methanol mixture in a direct methanol fuel cell. Reactants are directed to the fuel cell electrodes and are distributed to catalyst therein by means of fluid diffusion layers.
SPE fuel cells employ a membrane electrode assembly (MEA) which contains the solid polymer electrolyte or ion-exchange membrane disposed between the two electrodes. Each electrode contains a catalyst layer, comprising an appropriate catalyst, located next to the solid polymer electrolyte. The catalyst may, for example, be a metal black, an alloy or a supported metal catalyst, for example, platinum on carbon. The electrodes typically also contain a porous substrate (e.g., a porous electrically conductive sheet material) that may be employed for purposes of mechanical support and/or reactant distribution, thus serving as a fluid diffusion layer. For a gaseous reactant, the fluid diffusion layer is known as a gas diffusion layer (GDL).
The MEA is typically disposed between two plates to form a fuel cell assembly. The plates act as current collectors and provide support for the adjacent electrodes. The assembly is typically compressed (e.g., of order of 70 psi overall) to ensure good electrical contact between the plates and the electrodes, as well as to effect sealing between fuel cell components. A plurality of fuel cell assemblies may be combined in series or in parallel to form a fuel cell stack. In a fuel cell stack, a plate is usually shared between two adjacent MEAs, and thus also serves as a separator to fluidly isolate the fluid streams of the two adjacent MEAs.
Flow fields are typically incorporated into both surfaces of such plates in order to direct reactants across the electrochemically active surfaces of the fluid diffusion electrodes or electrode substrates. The flow fields typically comprise fluid distribution channels separated by landings. The channels provide passages for the distribution of reactant to the electrode surfaces and also for the removal of reaction products and depleted reactant streams. The landings act as mechanical supports for the fluid diffusion layers in the MEA and provide electrical contact thereto. Ports and other fluid distribution features are typically formed in the surfaces at the periphery of such flow field plates. When assembled into a fuel cell stack, the stacked ports can form internal manifolds for distribution of the fluids throughout the stack. The other distribution features typically are provided to distribute fluids from the ports to the appropriate flow fields.
Numerous seals are required in a typical SPE fuel cell stack. For example, seals are typically required around the edges of the various ports, MEAs, and flow field plates in order to appropriately isolate the different fluids within the stack and in order to prevent external leaks. Large cell stacks can comprise hundreds of cells and consequently many hundreds of seals. It is important therefore to employ highly reliable seal designs. However, obtaining highly reliable seals is a continuing challenge. In order to obtain ever greater power density, the trend is to employ the thinnest cells possible in the fuel cell stacks. In turn, this means that the seals employed become thinner, thereby aggravating tolerance stack-up issues. That is, the thinner the seal becomes, the wider the range of compression experienced for any given stack-up tolerances. Thus, either seals must be capable of tolerating greater ranges of compression (e.g., by using multiple seals designed to accommodate different ranges of compression) or ever tighter tolerances are required on the thickness of the cell components.
Many of the seals required in a SPE fuel cell stack can conveniently be integrated into the MEA assemblies as disclosed in U.S. Pat. No. 6,057,054. In such assemblies, seals are integrated at the edge of the MEAs by impregnating the porous electrode layers on either side. The seal extends laterally beyond the edge of the MEA and envelops its periphery. Such a seal can prevent fluid transfer around the edge of the MEA and can also be used to effect fluid tight seals to both adjacent flow field plates. Additional seals for internal ports or manifolds may also be incorporated at the same time as the edge seal for the MEA using an appropriate molding operation. Such designs also allow for the use of flush-cut MEAs (in which membrane and electrodes are cut simultaneously from a larger laminate) and the unitary assemblies are convenient for assembly purposes. However, as mentioned above, highly reliable seals are required for SPE fuel cell stacks and other developments in stack design can place additional demands on the seals. Thus, improvements in seal designs are always being sought.